1. Field
The present invention relates to rail-to-rail operational amplifiers (op-amps). More particularly, the present invention selectively adds auxiliary frequency compensation, depending on the behavior of an output transistor of the op-amp.
2. Background
Today, virtually every electronic circuit contains one or more operational amplifiers (op-amps). Basically, an op-amp is used to amplify an input signal. General use of op-amps has been extended to include applications such as DC amplifiers, AC amplifiers, comparators, servo valve drivers, deflection yoke drivers, low distortion oscillators, AC to DC converters, multivibrators, and many other applications. As electronic circuits become more complex, performance requirements are becoming more stringent, leaving designers with more difficult specifications to comply with and less flexibility to meet those difficult specification requirements. For example, with the proliferation of wireless communication (e.g., cell phones, pdas, laptops, etc.), electronic devices are becoming smaller, lighter, and are typically run off of batteries. For battery powered electronic devices, low power consumption is of critical importance. Consequently, many of these electronic devices require op-amps to operate with low supply voltage and low quiescent current. Due to the stringent requirements of low voltage applications, op-amps are being specifically designed to make use of the entire dynamic range offered by the power supply. In other words, the output stage of these op-amps are driven towards the upper and lower supply voltages (the “rails”). This has lead to the design of what is commonly referred to as “rail-to-rail” op-amps.
Unfortunately, when the output stage of a rail-to-rail op-amp is driven towards the upper and lower supply voltages, the output bipolar transistors are prone to becoming “saturated.” When the output bipolar transistors become saturated, the entire frequency response of the op-amp is detrimentally affected. In extreme cases, the op-amp can be caused to enter oscillation. An oscillatory op-amp may render the electronic device unusable or otherwise result in malfunctions.
In order to stabilize the op-amp and otherwise prevent oscillation, some form of frequency compensation is typically implemented. FIG. 1 shows a typical prior art op-amp with compensation circuitry. Op-amp 101 is comprised of an input stage 102. Input stage 102 is typically a differential pair for accepting the positive and negative input signals. A gain stage 103 follows the input stage 103. Gain stage 103 provides high voltage gain. The output stage 104, which provides high current driving capability, is coupled to the gain stage 103. Additionally, in a rail-to-rail configuration, the output stage is also a voltage gain stage. A frequency compensation circuit 105 is added to provide the requisite frequency compensation.
When choosing the degree of frequency compensation to be implemented, the worst-case scenario is often taken into account. The worst-case scenario typically arises when one or both of the op-amp's output bipolar transistors is in saturation. In order to address this possibility, most frequency compensation circuits are designed to prevent the op-amp from entering oscillation, even if either or both of the output bipolar transistors become saturated. In other words, the op-amp is overcompensated in case the output bipolar transistors ever become saturated. Unfortunately, this results in the op-amp being overcompensated when it is running in its normal (i.e., non-saturated) mode of operation. This overcompensation is undesirable because it degrades the frequency response of the op-amp.
Thus, designers are faced with a dilemma. On the one hand, a bipolar rail-to-rail op-amp can be designed to consider the worst-case scenario, by compensating for it. The advantage of this approach is that the op-amp is guaranteed to operate in a totally reliable and predictable manner, in spite of saturated output transistors. The disadvantage to this approach is that when the op-amp operates normally, the overcompensation results in a degraded frequency response. On the other hand, a bipolar rail-to-rail op-amp can be designed with minimal frequency compensation. The op-amp can thereby run near or at its peak frequency potential under most normal conditions. However, the downside is that, should a situation arise which exceeds the compensation capability of that op-amp, the op-amp might cease to work properly.
This problem is also manifest in MOS output rail-to-rail op-amps as well as in the bipolar output rail-to-rail op-amp described above. However, in a MOS device, what is called “saturation” in a bipolar transistor, is referred to as the “linear” or “resistive” region. In this linear region, also encountered when the MOS op-amp output node voltage approaches one or the other supply potential, the MOS device's characteristics are heavily compromised.